1. Field of the Invention
The invention relates to downhole tools for use in wellbores, and more particularly, to such tools having a sliding valve for controlling fluid flow therethrough at the upper end thereof. These tools, such as packers, may have drillable components, such as the valve, therein made at least partially of non-metallic materials, such as engineering grade plastics.
2. Description of the Prior Art
In the drilling or reworking of oil wells, a great variety of downhole tools are used. For example, but not by way of limitation, it is often desirable to seal tubing or other pipe in the casing of the well, such as when it is desired to pump cement or other slurry down tubing and force the slurry out into a formation. It then becomes necessary to seal the tubing with respect to the well casing and to prevent the fluid pressure of the slurry from lifting the tubing out of the well. Packers designed for these general purposes are well known in the art, and valves for controlling fluid flow through the packers once the packers are set are also known.
When it is desired to remove many of these downhole tools from a well bore, it is frequently simpler and less expensive to mill or drill them out rather than to implement a complex retrieving operation. In milling, a milling cutter is used to grind the tool, or at least the outer components thereof, out of the well bore. Milling is a relatively slow process, but it can be used on tools having relatively hard components such as erosion-resistant hard steel. One such tool is the packer disclosed in U.S. Pat. No. 4,151,875 to Sullaway, assigned to the assignee of the present invention and sold under the trademark EZ Disposal packer.
In drilling, a drill bit is used to cut and grind up the components of the downhole tool to remove it from the well bore. This is a much faster operation than milling, but requires the tool to be made out of materials which can be accommodated by the drill bit. Typically, soft and medium hardness cast iron are used on the pressure bearing components, along with some brass and aluminum items. Tools of this type include the Halliburton EZ Drill.RTM. and EZ Drill SV.RTM. squeeze packers.
The EZ Drill SV.RTM. squeeze packer, for example, includes a lock ring housing, upper slip wedge, lower slip wedge, and lower slip support made of soft cast iron. These components are mounted on a mandrel made of medium hardness cast iron. The EZ Drill.RTM. squeeze packer is similarly constructed. The Halliburton EZ Drill.RTM. bridge plug is also similar, except that it does not provide for fluid flow therethrough.
Such drillable devices have worked well and provide improved operating performance at relatively high temperatures and pressures. Tools such as the packers and plug mentioned above are designed to withstand pressures of about 10,000 psi and temperatures of about 425.degree. F. after being set in the well bore. Such pressures and temperatures require the cast iron components previously discussed.
However, drilling out iron components requires certain techniques. Ideally, the operator employs variations in rotary speed and bit weight to help break up the metal parts and reestablish bit penetration should bit penetration cease while drilling. A phenomenon known as "bit tracking" can occur, wherein the drill bit stays on one path and no longer cuts into the downhole tool. When this happens, it is necessary to pick up the bit above the drilling surface and rapidly recontact the bit with the packer or plug and apply weight while continuing rotation. This aids in breaking up the established bit pattern and helps to reestablish bit penetration. If this procedure is used, there are rarely problems. However, operators may not apply these techniques or even recognize when bit tracking has occurred. The result is that drilling times are greatly increased because the bit merely wears against the surface of the downhole tool rather than cutting into it to break it up.
While cast iron components may be necessary for the high pressures and temperatures for which they are designed, it has been determined that many wells experience pressures less than 10,000 psi and temperatures less than 425.degree. F. This includes most wells cemented. In fact, in the majority of wells, the pressure is less than about 5,000 psi, and the temperature is less than about 250.degree. F. Thus, the heavy duty metal construction of the previous downhole tools, such as the packers and bridge plugs described above, is not necessary for many applications, and if cast iron components can be eliminated or minimized, the potential drilling problems resulting from bit tracking might be avoided as well.
Some embodiments of the downhole tool of the present invention solve this problem by providing an apparatus wherein at least some of the components, including pressure bearing components, are made of non-metallic materials, such as engineering grade plastics. Such plastic components are much more easily drilled than cast iron, and new drilling methods may be employed which use alternative drill bits such as polycrystalline diamond compact bits, or the like, rather than standard tri-cone bits.
The Halliburton EZ Drill SV.RTM. squeeze packer has a pressure balance sliding valve for control of fluid movement in the well. The valve is disposed in a center mandrel of the packer. The valve is operated by reciprocation of the tubing, and may be opened and closed, as desired, before and after squeeze cementing. Some of the embodiments of the present invention also utilize a sliding valve within the mandrel, but differ in the use of non-metallic components.
Although the EZ Drill SV.RTM. configuration with the valve disposed in the mandrel has worked well, it does require machine work on the inside of the mandrel. This would also be true on the non-metallic embodiments disclosed herein. Of course, any machining adds to the cost of the components. Also, the valve itself which slides inside the mandrel reduces the flow area through the packer, thus causing at least some restriction to the inside of the packer mandrel. Thus, there is a need for a downhole tool, such as a packer, with less flow restriction therethrough and one in which the inner surface of the mandrel requires no machining. Further, in order to operate a packer of the EZ Drill SV.RTM. configuration, it is necessary to run a stinger into the tool to actuate the valve. In some wells, proper insertion of the stinger may be difficult, and therefore elimination of the need for a stinger is desirable in these cases.
A preferred embodiment of the present invention solves these problems by utilizing a valve disposed on the outside of the mandrel so that there is no need for machining the inside surface of the mandrel. Because the valve is on the outside of the mandrel, there is no necessity for a stinger. Rather, a simpler-to-operate overshot is used to actuate the valve. This particular embodiment of the invention may be utilized for downhole tools with metallic components as well as non-metallic components. The elimination of machining in the mandrel is particularly important in tools wherein the mandrels are made of non-metallic materials, because the mandrel may be fabricated by molding the inside diameter to size. Such a molding process would create a sufficiently smooth finished internal diameter when there is no sliding valve disposed therein.
Another problem with packers having the valve below the packer element is that this results in pressure being held at the bottom of the packer. When drilling out the packer, the upper slips which keep the packer from moving upwardly are drilled first and released from engagement with the well bore before the pressure is relieved therebelow. This can result in the packer being forced upwardly by pressure acting thereon which can cause jamming of the drill bit or can cause the entire tool string to move up the well bore. To avoid this, it is necessary to open the valve before the drilling process which may not always be desirable.
The preferred embodiment using the valve on the outside of the mandrel also preferably positions the valve above the packer elements. In this way, as the packer is drilled out, the valve is drilled before the slips and packer elements. Thus, pressure is relieved while the packer is still held in the well bore by the slips. Thus, no pressure surge can result in a portion of the tool being forced upwardly.